Automatic volume control system



April 30, 1940.

N. F` sHoFsTALl. 2,199,350

AUTOMATIC VOLUME coNTRoL sTsTEM Filed Dc. s, 193e.

Invent or Norton T- Shofstal I,

Patented pr. 30, 1940 L' UNITED STATES AUTOMATIC VOLUME CONTROL SYSTEM Norton F. Shofstall, Stratford, Conn., assigner to General Electric Company, `a, corporation of New York Application December 8, 1938, Serial No. 244,621

13 Claims.

. My invention relates to automatic volume control systems for signaling apparatus, such as radio receivers, and it has for one of its objects to eifect certain improvements therein with respect to theavoidance of overloading of the controlled stages of the apparatus.

My invention relates more particularly to automatic volume control systems in which the control potential for regulating the gain of the various stages of the receiver is supplied thereto through Vindividual circuits employing the usual filter resistors for suppressing audio and radio potentials which appear on those conductors.

One of the objects of my invention is to reduce overloading produced by reason of grid current flowing in these resistors.

Another object of my invention is to reduce the effects of such overloading through redistribution of the control potentials supplied to regulate the gain of the several amplifier stages employed in the receiver.

A further object of my invention is to provide an automatic volume control arrangement inv which the receiver does not cut-off as the tuning of the receiver is adjusted to proximity with a strong signal and one which, therefore, allows the automatic frequency control system of the receiver to function in normal manner accurately to tune the receiver to a desired signal Whether the signal be relatively weak or very strong.

The novel features which I believe to be characteristic of my invention are pointed out with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. l represents an embodiment of my invention and Fig. 2 represents a modification thereof.

Referring particularly to Fig. 1 of the drawing, my invention is illustrated as embodied in a superheterodyne type of radio receiver in which an antenna ground system I0, Il is connected to the input circuit of a tunable radio frequency amplifier, represented conventionally by the rectangle I2 which may comprise one or more stages of amplification. The output of the radio frequency amplifier I2 is supplied to additional tunable stages of amplification, represented conventionally by the rectangle I3, which may include a converter for changing the oscillations- 55 of signal frequency to oscillations ofl intermedi- (Cl. Z50-20) ate frequency and one or more stages of inter- -mediate frequency amplification.

The output of the last stage of intermediate frequency amplification included in the apparatus I3 is supplied to a frequency discriminatory vnetwork comprised by a transformer lli having a primary winding I5 and a center-tapped secondary winding it. The primary winding I5 and the secondary winding I6 are tuned to the intermediate frequency by the respective condensers I'I and I8.

Each end of the transformer secondary winding `Il is connected to an anode element I9, of a respective diode rectifier 2I, 22. The diodes 2l land 22 .have cathode elements 23, l2li which are connectedtogether through a load resistor 25. A radio frequency choke 26 connects the center-tap of the secondary winding I6 with a center-tapon the resistor 25. A condenser 2l connects the high potential end of the transformer primary winding I5with thecenter-tap of the transformer secondary winding I6. The cathode 24' of the diode 22 is connected directly to ground as shown while the cathode 23 of the diode 2| is maintained at ground-potential for currents of intermediate and audio frequency by the use of a condenser 28.

The potential appearing across the lower half of the load resistor 25 is supplied through a condenser 29 to an audio frequency amplier, represented conventionally by the rectangle 30, which may includewone or more stages of audio frequency amplification. IThe output of the audio frequency amplifier is supplied to a translating device 3| hereshown by way of illustrationr as a loud speaker.

The connection of the transformer I4 in the manner shown provides a frequency discriminatory network whose operation is explained in detail in the' October 1935 issue of the Proceedings of the I. R,E'. beginning at page 1125. An automatic frequency control potential appears between ground and the upper terminalof the resistor 25. This potential is supplied through an electrical conductor 32 to an automatic frequency control tube, included in the apparatus I3, which operates in a manner'well-known in the art tol maintain substantially constant the oscillations supplied to the intermediate frequency amplier.

A portion of the voltage appearing across the lower half -of theresistor 25 is supplied through the conductors V33 and 34 and through a filter comprised by a resistor 35 and a condenser 36 to the control electrode of one or more of the several electron discharge devices included in the converter stage and the intermediate frequency amplifier stages of the aparatus I3. This automatic volume control potential operates in wellknown manner to maintain substantially constant the output of the intermediate frequency amplifier though the input signal strength may change over a wide range of values. A larger proportional part of the potential appearing across the lower half of the resistor is supplied through the conductors 31 and 38 and through a filter comprised by a resistor 39 and a condenser 48 to the control electrode of one or more of the electron discharge devices included in the radio frequency amplifier stages of the apparatus l2. The value of this automatic volume control potential is chosen several times larger than that supplied to the converter and intermediate frequency amplifier for the purpose of preventing the overloading of the radio frequency amplifier and converter stages when the receiver is tuned exactly to resonance with a very strong signal which may, for example, be of the order of two volts or more at the antenna input terminals.

The automatic volume control system thus far considered performs meritoriously as long as the receiver is accurately tuned to very close resonance with a signal. However, as soon as the receiver is slightly detuned from or is initially tuned only to proximity with a strong signal, the highly selective circuits of the intermediate frequency amplier greatly reduce the automatic volume control potential appearing in the conductors 34 and 38. This permits the radio frequency amplifier I2 to operate in overloaded condition at high gain and causes the converter, and such other of the radio frequency stages of amplication as precede the converter and are biased through the conductor 34, thereupon to overload by having its control grid driven into the positive bias region where the grid draws current. The grid current flows through theresistor 35 and the lower portion of the resistor 25 to produce across these resistors a potential drop whose polarity is in a direction further to reduce the gain of the intermediate frequency stages of amplification by driving them toward cutoff. Little or no signal energy thereafter passes to the output of the intermediate frequency amplifier and little or no. automatic frequency control or automatic volume control potentials are developed across the resistor 25. This renders the automatic frequency control arrangement so unresponsive that the latter in effect is deenergized and the receiver fails to pullin to the strong signal.

To overcome this condition of operation, advantage is taken of the fact that the converter normally overloads more than the radio frequency stages of amplification. A diode rectifier 42 is connected between the automatic volume control conductors 34 and 38, the anode element 4| of the rectifier being connected to the automatic volume control conductor 38 while the cathode element 43 is connected through a resistor 44 to the automatic volume control conductor 34.

' With the diode rectifier 42 connected in this manner, the relatively small radio frequency amplier grid current and the relatively larger converter control grid current which flows when the receiver is tuned only to proximity with a strong signal makes the automatic volume control conductor 34 negative with respect to the automatic volume control conductor 38 thereby to render the diode anode 4! positive relative to the diode cathode 43. Grid current which previously flowed through the resistor 35 now flows from ground to the converter control grid through a series circuit including the resistors 25 and 39, the conductor 38, the diode rectier 42 and the conductor 34. This decreases the negative bias produced across the resistor 35 and increases the negative potential of the automatic volume control conductor 38 thus biasing the radio frequency amplier included in the apparatus I 2 toward cut-off to limit its output and thereby reduce the overload on the converter. The diode 42 has a relatively low anode to cathode resistance and, as long as the diode passes current, the conductors 34 andv 38 are effectively tied together electrically and operate at substantially the same potential above ground. This prevents the intermediate frequency amplifier from being biased to or toward cut-off and allows the generation of some automatic frequency control potential and automatic volume control potential. The automatic frequency control system thereupon operates to change the tuning of the oscillatory circuit of the converter more accurately to tune the receiver to resonance with the strong signal.

When this occurs, a larger automatic volume control potential is developed across the resistor 25 and the gain of the radio frequency amplier and converter is reduced. This removes the overload on the radio frequency amplifier and converter thereby to improve the fidelity of reproduction of the received signal.

The cathode 43 of the diode 42 is normally energized with alternating current. The resistor 44 is inserted between the conductor 34 and the diode 42 to form, in conjunction with the condenser 36, a ripple filter for the purpose of reducing alternating current pulsations which might otherwise be induced on the automatic volume control conductors 34 and 38 by the alternating current energization of the cathode 43.

It has been found that without the diode 42 connected in circuit, the automatic frequency control tube would not cause the receiver to pull-in to resonance with a strong 1400 kc. signal (having a strength of two volts at the antenna) whenever the receiver was tuned to more than about l0 kc. off resonance. With the diode 42 connected as shown, the same receiver would pull in to the same signal when the receiver was tuned as much as 20 kc. off resonance. 'Ihe failure of a receiver to pull in to resonance with a selected signal is particularly serious in the increasingly popular motor tuned or trimmer condenser tuned type of radio receiver where a person tunes the receiver by merely pushing a selected push-button. The mechanical error in electric motor tuning may be 5 or 10 kc. which, in the absence of my invention, is sucient to render the receiver inoperative to receive very strong local stations. This condition of operation applies in like manner to trimmer condenser tuning where the condensers are improperly adjusted or, having been properly adjusted, have drifted somewhat from their initial setting.

The several ampliers of the Fig. 1 arrangement areself-biased in a manner well known in the art by the use of a suitable cathode biasing resistor and parallel connected condenser, not shown. In Fig. 2, I have shown how my invens tion may be used where it is desirable to provide some control of the sensitivity of the radio recever.` The Fig. 2 arrangement .dispenses with the necessity of biasing the amplierstages in their cathode circuit and allows the cathodes of the several amplifier electron discharge devices to be directly connected to ground. Elements in this gure corresponding to like elements of Fig. l are designated by like reference characters.' The apparatus represented conventionally by the rectangle of this gure includes, in addition to the apparatus I3 of Fig. 1, the frequency discriminatory network of Fig. lcomprised by the transformer I4 and interconnected circuit, and the diode rectiiiers 2I and 22.

In the Fig. 2 embodiment, two diode rectiers are included in a common envelope 46. One of the diode anodes 41 is connected to the automatic volume control conductor 34 while' the second diode anode 48 is connected to the automatic volume control conductor 38. A source of substantially constant unidirectional potential 49, 50 is connected across a voltage divided resistor 5I, a point 49 of which is grounded. `A supplementary voltage divider resistor 52 is connected upon closure of a manuallyoperated switch 53 across a portion of the resistor'5l. The cathode 54 of the electron discharge device 46 is connected to a point on the resistor 5I negative with respect to ground. The cathode 55 of the device-46 is connected to a point on the resistor 52.

The unidirectional potential source 49, 5|) produces a iiow of current through two circuits, the first of which may be traced from the terminal 49 through ground, through the lower portion of the resistor 25, the conductor 31, the resistor 39, the diode anode 48, the diode cathode 54, and through a conductor 56 to a point on the resistor 5I negative with respect to the terminal 49. The second circuit may be traced from the terminal 49 through ground, through the lower portion of the resistor 25, the conductor 33, the resistor 35, the diode anode 41, the diode cathode 55, and through the left-hand portion of the resistor 52 to a point on the resistor 5I negative with respect to the terminal 49. The current flowing in each of these two circuits produces a potential drop across the resistors 35 and 39 thereby to provide a normal operating bias for the control electrodes of the several amplifier stages included in the apparatus I2 and 45.' 4The switch 53 when closed increases the current flow through the resistor 35 thereby to bias the amplier stages included in the apparatus 45 toward cut-off for the purpose ofk reducing the sensitivity of the radio receiver. i

The diode 42 in Fig. 2 operates in the manner of and for the same purpose as the corresponding diode in the Fig. 1 arrangement. It will be apparent that the diode rectifiers 41, 55 and 48, 54 serve only to provide a normal operating bias for the amplifiers. I2 and 45. Current flow through either of these rectifiers ceases when its anode becomes negative with Irespect to its cathode as would occur during the times when more than a relatively small automatic volume control potential is developed across the resistor 25 or when the grid current through the resistors 35 and 39 increases above a predetermined value.

It will be evident from the preceding description of my invention and its operation that by the term strong signal I refer to any signal whose strength is such that at the location of the receiver it causes the grid of a radio frequency amplifier tube or a converter tube or both to draw grid current when the receiver is tuned to some frequency within afrequen'cy range on either side of and including the signal.

4While I have illustrated particular embodil. The combination, in a radio receiver, of aplurality of amplification stagesconnected in cascade, means automatically to reduce the'ampliiication of said stages in response to increasing intensities of signal currents f supplied thereto, and means responsive to theover-loading of a later one of said stages in the.l cascade connection to further reduce thev amplification of an earlier stage in the cascade connection.

2. The combination, in a radio receiver, ofk a plurality of amplifier stages connected in cascade, mans automaticallyfto reduce the amplification of said stages in response to theincreasing intensity of a received signal, said vmeans more greatly reducing the amplification of earlier stages than later stages in said cascade connection, and means responsive to the over-loading of a later stage for further reducing the amplification of an earlier vstage.`

3. In a. radio receiver, the combination Iof a radio frequency amplifier, a frequency' converter, and an intermediate frequency amplifier connected in cascade in the order named, means automatically to reduce the gain of each of -said amplifiers in response to the increasing intensity of a received signal, and means responsive .to the overloading of said converter for reducing the gain of said radio frequency amplifier.

4. In a radio receiver, the combination of a radio frequency amplifier, a frequency converter, and an intermediate frequency amplifier connected in cascade in the order named; meansautomatically to reduce the gain of each of said amplifiers in response to the increasing intensity of a received signal, a unilateral current ,con-

5. In a radio receiver, the combination of a radio frequency amplifier, a frequency converter, andan-intermediate' frequency amplifier connected in cascade in the order named, means automatically to reduce the gain of each of said amplifiers in response to the increasing intensity of a received signal, said means more greatly reducing the gain of said radio frequency amplifier than said intermediate frequency amplier, a unilateral current conducting device, and means responsive to the overloading of said converter for causing said device to become conductive to reduce the gain of said radio frequency amplifier. i

6. The combination in a radio receiver, of a radio frequency amplifier, a frequency converter, and an intermediate frequency amplifier connected in cascade in the order named, means automatically to reduce the gain `of each of said amplifiers and of said converter in response to the increasing intensity of a received signal, said means more greatly reducing the gain of Said radio frequency amplifier than said intermediate frequency amplifier, a unilateral current conducting device, and means responsive to the overloading of said converter forcausing said device to become conductive thereby equally to reduce the gain of each of said amplifiers and said converter.

7. The combination, in an automatic volume control system, of a plurality of electron discharge stages connected in cascade, each of said stages having an amplification control electrode, a rectifier connected to the output of the last of said stages, individual circuits connected betw-een the output of said rectiiier and each of' said amplification control electrodes whereby the potential on each of said control electrodes is controlled by said rectifier, a resistor in each of said circuits, and means responsive to current flowing in one of said resistors to interconnect said circuits thereby to pass a portion of said current through the resistor of the other circuit whereby the potential produced by said current flowing in said other resistor is effective on the control electrode of the stage to which said rey sistor is connected.

8. The combination, in an automatic volume control system, of a plurality of electron discharge stages connected in cascade, each of said stages having an amplification control electrode,

a rectifier connected to the output of the last of said stages, individual circuits connected between the output of said rectiiier and-each of said control electrodes whereby the potential on each of said electrodes is controlled by said recp tiiier, a resistor in one of said circuits, and means `to its respective cathode than the other grid is biased negatively with respect to its respective cathode, and means responsive to increase in intensity in said amplified currents beyond a predetermined value to equalize the bias voltage on said grids with respect to said cathodes.

l0. In combination, a plurality of cascade connected amplifiers having a control electrode and a cathod-e, means to rectify the output from the last of said amplifiers and to supply a portion of the rectified potential between the grid and cathode of each of said plurality of amplifiers, the potential supplied between the grid and cathode of one amplifier being greater than that supplied between the grid and cathode of a different of said amplifiers, and means to equalize said last two-mentioned voltages in response to increase in yintensity of current amplified by said ampliers beyond a predetermined value.

11. In combination, a plurality oi cascade connected amplifiers, each of said amplifiers having a control electrode and a cathode, means to rectify the output from the last of said amplifiers and to supply the rectified potential to a resistor, conductors extending from different points on said resistor to eachof said grids, and means including said conductors to bias each grid negatively with respect to its respective cathode, one grid being biased more negatively than another, and means responsive to current in one of said conductors to connect said conductors together thereby to equalize the voltage on the corresponding grids.

l2. In combination, a plurality of cascade connected amplifiers, each of said amplifiers having a control electrode and a cathode, means to rectify the output from the last of said amplifiers and to supply the rectified potential to a resistor, conductors extending from points of different potential on said resistor to corresponding grids of said amplifiers, a diode connected between said conductors and so poled with respect to the potential between said conductors that it is normally non-conductive, and means to render said diode .conductive in response to current in one of said grids.

13. In combination, a plurality of cascade connected ampliiiers, each of said amplifiers having a control electrode and a cathode, means to rectify the output from the last of said amplifiers and to supply the rectified potential to a resistor, conductors extending from points'of different potential on said resistor to corresponding grids of said ampliers, a diode connected between said conductorsand so poled that it is normally nonconductive, resistances connected in the respective conductors between said diode and said firstmentioned resistor whereby when current liows in the less negative of said conductors due to grid conduction in the respective amplifier, the cathode of said diode becomes negative relative to its anode, thereby reducing the current in the resistance in said one conductor and increasing current in the resistance in the other conductor thereby increasing the negative voltage on the other grid. y

' NORTON F. SHQFS'IALL. 

